The 5-HT3 antagonists are a class of drugs that act as receptor antagonists at the 5-HT3 receptor, a subtype of serotonin receptor found in terminals of the vagus nerve and in certain areas of the brain. With the notable exception of alosetron and cilansetron, which are used in the treatment of irritable bowel syndrome, all 5-HT3 antagonists are antiemetics, used in the prevention and treatment of nausea and vomiting. They are particularly effective in controlling the nausea and vomiting produced by cancer chemotherapy and are considered the gold standard for this purpose. Ondansetron is a serotonin 5-HT3 receptor antagonist used alone or with other medications to prevent nausea and vomiting, and is used for preventing nausea and vomiting caused by cancer drug treatment (chemotherapy) and radiation therapy. It is also used to prevent and treat nausea and vomiting after surgery.